A commonly employed process for producing terephthalic acid involves the air oxidation of p-xylene in hot acetic acid, usually with a cobalt catalyst. A slurry of terephthalic acid in acetic acid is produced. The terephthalic acid precipitate is filtered off and dried, while the acetic acid filtrate is returned to the oxidizer. The terephthalic acid thus produced can be subsequently converted into polyethylene terephthalate and spun into fibers using well-known methods.
A problem associated with this process for producing terephthalic acid is that the p-xylene feed normally contains some varying amount of m-xylene, which is oxidized to isophthalic acid and is removed along with the terephthalic acid. These residual mixtures cannot be separated by fractional distillation because of the close high boiling points of isophthalic and terephthalic acids. Also, because of their high boiling points, there is the problem of decomposition and discoloration of the products. In addition, terephthalic acid has a tendency to sublime, resulting in the plugging of distillation equipment. Thus, more complicated separation procedures than simple fractionation have been proposed to separate isophthalic acid and terephthalic acid.
For example, British Patent Specification No. 970,781 discloses a method for separating isophthalic acid from terephthalic acid which comprises leaching a mixture of terephthalic acid and isophthalic acid with a solvent at an elevated temperature above the boiling point of the solvent and at a superatmospheric pressure in order to maintain the solvent in the liquid phase. An amount of solvent is employed which is sufficient to dissolve substantially all of the isophthalic acid, thereby also forming a saturated solution of the less-soluble terephthalic acid and a slurry of undissolved terephthalic acid. The slurry is thereafter thickened with respect to terephthalic acid by removing from the slurry a substantial portion of the solution containing the dissolved isophthalic acid and terephthalic acid. The thickened slurry of terephthalic acid is thereafter washed with a liquid at elevated temperature and superatmospheric pressure in order to substantially replace the remainder of the solution and to form a second slurry of solid terephthalic acid, which is thereafter separated from the second slurry. The disclosed process involves numerous process steps, thereby involving the consumption of time and energy.
U.S. Pat. No. 2,820,819 discloses that mixtures of isophthalic acid and terephthalic acid can be separated by intimately mixing the acids with liquid water at temperatures in the range of 350.degree.-500.degree. F. Under these conditions, a slurry is formed consisting of a concentrated solution of isophthalic acid and a solid phase which is predominantly terephthalic acid. The slurry is filtered at 350.degree.-500.degree. F. to obtain a filter cake, which has a substantially higher terephthalic acid content than the initial mixture, and a filtrate containing in solution a mixture of isophthalic acid and terephthalic acid having a much higher isophthalic content than the initial mixture. This process involves the utilization of elevated temperatures which require the consumption of large quantities of energy, thereby increasing the cost of production of terephthalic acid. In addition, it can be seen from the Example that the disclosed process yields a product which still contains approximately 2% isophthalic acid.
The prior art also discloses the use of extraction techniques for the separation of isophthalic and terephthalic acids. For example, U.S. Pat. No. 2,840,604 discloses a method for separating benzoic acid, isophthalic acid, and terephthalic acid which involves extraction (preferably, an initial extraction followed by a subsequent final extraction) with a solvent such as lower aliphatic alcohols, ketones, and carboxylic acids. Specific examples include methanol, ethanol, normal propanol, isopropanol, tertiary butanol, acetone, methyl ethyl ketone, diethyl ketone, methyl isobutyl ketone, formic acid, dioxane, and tetrahydrofuran.
U.S. Pat. No. 3,059,025 discloses the separation of isophthalic acid and terephthalic acid by extraction with pyridine.
U.S. Pat. No. 3,080,420 discloses a separation process whereby an ammonium solution of mixed phthalic acids is contacted with activated carbon and is then added to a solution of an acid having an ionization constant greater than that of terephthalic acid or isophthalic acid (e.g., sulfuric acid) in order to regenerate and precipitate the acid.
Unlike certain of the prior art processes discussed above, the process of the present invention does not require elevated temperatures or multiple process steps. The present invention provides an inexpensive, facile means of separating isophthalic acid from terephthalic acid, thereby yielding a terephthalic acid product of improved purity. In addition, the effect of m-xylene fluctuations in the feed to the oxidizer upon the final isophthalic content in the terephthalic acid product is strongly dampened.